In recent years, attention is paid on a semiconductor optical device formed on an SOI (Silicon on Insulator) substrate. The semiconductor optical device includes a first clad, a second clad and an optical waveguide layer sandwiched between the first clad and the second clad. The optical waveguide layer includes an i-type core, an n-type slab portion which is thinner than the core and disposed on one side thereof, and a p-type slab portion thinner than the core and disposed on the opposite side of the core from the n-type slab layer.
As such, the semiconductor optical device formed on the SOI substrate is an optical waveguide device including a p-i-n homojunction (hereafter referred to as homojunction optical waveguide device). (For example, refer to Japanese Laid-open Patent publication No. 2004-325914; L. Naval, R. Jalali, L. Gomelsky, and J. M. Liu, “Optimization of Si1-xGex/Si Waveguide Photodetectors Operating at 1.3 um”, Journal of Lightwave Technology, Vol. 14, pp. 787-797, 1996; and Chris G. Van de Walle and Richard M. Martin, “Theoretical calculations of heterojunction discontinuities in the Si/Ge system”, Vol. 34, pp. 5621-5633, 1986.)
Carriers are injected into the i-type core when a voltage is applied between the p-type slab portion and the n-type slab portion of the homojunction optical waveguide device. This produces a variation in the refractive index and the loss coefficient of the core. Accordingly, by varying a voltage applied between the p-type slab portion and the n-type slab portion, it is possible to vary the phase and the intensity of light which propagates through the core (hereafter referred to as propagating light).
However, since there is no barrier in the p-i-n homojunction to retain the injected carriers within the junction portion, it is difficult to obtain a high carrier density in the junction portion. To cope therewith, in the homojunction optical waveguide device, the phase or the intensity of the propagating light is varied to a desired value by injecting a large amount of currents into the p-i-n homojunction. Alternatively, the phase or the intensity of the propagating light is varied to a desired value by elongating the device length. As a result, there is a problem in the homojunction optical waveguide device that power consumption becomes large or the device length becomes long.